1. Field of the Invention
The present invention relates to an improved socket assembly for accommodating an electronic or electrical component such as an integrated circuit chip or a package (IC) or a chip-on-board module etc. and for electrically connecting an electrical component to a printed circuit board.
2. Description of the Related Art
In general, an electronic or electrical component such as an integrated circuit chip or package (IC) or a chip-on-board module etc. is connected and affixed by directly soldering terminals such as leads or pads arranged along the sides of the electrical component onto the printed circuit board. On the other hand, where it is mounted on a printed circuit board for testing or the like in a manner enabling the electrical component to be easily replaced or where an electrical component in which the heat caused when soldering is liable to exert an adverse influence upon the internal circuits is mounted on a printed circuit board etc., generally use is made of a socket of a top loading type to accommodate the electrical component. As disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-3977 and Japanese Unexamined Patent Publication (Kokai) No. 2-51882, a conventional typical top load socket is provided with a generally rectangular configuration support frame or socket body made of a plastic. The socket body has a plurality of contact pins disposed in a row in parallel on each of the sides thereof. Each contact pin is provided with an external connecting portion for connection to a conductor pattern on the printed circuit board and a contact portion for coming into contact with a terminal of the electrical component. The socket body is further provided with a cap or a cover pressing against an upper surface of the electrical component mounted on the contact portions of the contact pins and an engagement piece for engaging the cover with the socket body at a position where the cover presses against the upper surface of the electrical component. The row-forming contact pins are defined in their intervals by partition walls or ribs provided in a row in parallel on the socket body. Further, the contact portions of the contact pins pass through a base plate of the socket body disposed on the printed circuit board and project beneath the same.
In the above-mentioned conventional socket, all terminals of the electrical component and all contact portions of all contact pins on the socket body are simultaneously brought into press-contact with each other by the pressing force of the cover and therefore when the number of the contact pins is increased, a very large pressing force becomes necessary so as to give a required contact pressure to the contact portions of the contact pins. Accordingly, an operation such as attachment, engagement, etc. of the cover becomes difficult. Accordingly, it is difficult to deal with higher density terminals in an electrical component such as an IC. Also, the electrical component is pressed from the top by a large force, and therefore this has become a cause of causing excessive stress in the electrical component or socket body. Also, if the strength of the socket body or cover is not sufficient, warping is produced in them, and as a result, the contact pressure of the contact pins becomes small, which becomes a cause of inducing poor contact. Particularly, in a high temperature environment, the plastic of the socket body undergoes stress relaxation, which further increases the warping.
Further, in the above-mentioned conventional socket, since the terminals of the electrical component and the contact portions of the contact pins are electrically connected by the contact pressure, poor conduction is apt to occur due to dirt, oxide film, etc. deposited on the terminals or contact portions. Accordingly, desirably, when the terminals of the electrical component come into contact with the contact portions of the contact pins, they should cause a so-called wiping action to remove the dirt, oxide film, etc. In the above-mentioned conventional socket, however, the electrical component positioned by the socket body is pressed in its thickness direction by the cover, and therefore it is difficult for a wiping action to occur between the electrical component and contact pins. Accordingly, it is difficult to enhance the reliability of the electrical connection between the electrical component and the contact pins.
A top load socket disclosed in U.S. Pat. No. 4,993,955 is provided with a plurality of contact pins arranged so as to form rows in parallel respectively along the sides of the support frame or the socket body and cams arranged along the sides of the socket body. Each contact pin has a curved arm portion and a contact portion formed at the free end of the arm portion for coming into contact with the upper surface of a terminal of the IC. In this socket, the contact pin can be operated by a cam for each row. However, the contact portions of the contact pins are brought into press-contact with the upper surface of the terminals of the IC due to the elastic force of the contact pins, and therefore the contact pressure between the contact portions of the contact pins and the IC depends on the spring constant of the contact pins. Also, since the contact pins press against the terminals of the IC from the top, hardly any wiping action occurs between the contact portions of the contact pins and the terminals of the IC.
Further, in the above-mentioned conventional socket, the partition walls or ribs defining the intervals in the row-forming contact pins are provided in the socket body so as to be aligned in a row. In this case, so as to reduce the pitch of the contact pins, it is necessary to use thin ribs. However, if the ribs are made thin, the strength thereof is lowered, and the shaping becomes difficult. Accordingly, there is a limit to making the pitch of the contact pins narrower.
Further, in the above-mentioned conventional socket, the socket body is placed on the printed circuit board and sandwiched between a part of the contact pins and the printed circuit board, and therefore in a state where the contact pins are soldered on the printed circuit board, the socket body cannot be detached from the printed circuit board. Also, the contact portions of the contact pins are positioned above the base plate of the socket body and the electrical component is mounted on the contact portions of the contact pins. Accordingly, there is a limit in suppression of the height of the electrical component mounted on the printed circuit board via the socket.